The present invention relates in general to operating a variable reluctance motor and more specifically to reliably starting and accelerating to rated speed a variable reluctance motor operating without a shaft position sensor.
Variable reluctance motors have poles or teeth on both the stator and the rotor (i.e. they are doubly salient). There are windings on the stator but no windings on the rotor. Each pair of diametrically opposite stator windings is connected in series to form one phase of the motor.
Torque is produced by switching current on in the stator phases in a predetermined sequence so that a magnetic force of attraction results between a pair of rotor poles and the stator poles that are being energized. The switched reluctance motor is a variable reluctance motor in which the current is switched off in each pair of stator windings at the commutation point before the approaching rotor poles rotate past the aligned position.
Each time a phase of the switched reluctance motor is switched on by closing a switch in a power converter, current flows in the pair of stator windings, providing energy from a DC supply to the motor. The energy drawn from the supply is converted partly into mechanical energy, by causing the rotor to rotate towards a minimum reluctance configuration, and partly into a stored magnetic field. When the switch is opened, part of the stored magnetic energy is converted to mechanical output and the remainder of the energy is preferably returned to the DC souce.
The converter must switch the phase currents on and off in synchronism with rotor position. This "shaft-position switching" is normally accomplished using a shaft position sensor by referencing the switching of the transistors in each converter leg to a set of pulses derived from the shaft position sensor. One example of a shaft position sensor is a fixed light source and a fixed light detector on opposite planar sides of a slotted disc connected to the shaft which optically interrupts the light beam between the source and the detector in accordance with the position of the shaft. The shaft position sensor is undesirable in small motors because of its cost, and in both large and small motors because of its space requirement and the vulnerability of the signal wires that must run between the motor and the electronic power converter.
Copending application Ser. No. 699,537, teaches that the average dc link current supplied to the power converter is substantially proportional to load torque and may be used as a feedback variable for controlling the motor (i.e. closed-loop control) without shaft position sensing. However, a controller which regulates averaged quantities will only work when the motor is running substantially at rated speed. Therefore, additional means are needed to start the motor.
It is known from variable reluctance stepper motor theory that a motor will follow (i.e. be started from) a fixed-rate stepping sequence from standstill provided that the stepping rate does not exceed a limit known as the starting rate, and further provided that the stepping rate does not coincide with a mechanical resonance of the drive system. The magnitude of the starting rate depends on the particular motor used, supply voltage, load and temperature.
Design procedure for open-loop stepper motors is to determine the starting rate and then to start the motor at a stepping rate well known below the starting rate to allow for changes in the parameters determining starting rate. The designer must also insure that the chosen stepping rate does not coincide with a resonant mechanical frequency of the system. For a stepping motor whose step angle is small compared to its rotor tooth pitch, stepping rates near a resonant mechanical frequency will result in a position error without the motor losing synchronism, a relatively minor problem. However, some types of variable reluctance motors (including switched reluctance motors) have a step angle that is a large fraction of its rotor tooth pitch (e.g. 1/3). These motors can easily lose synchronism if the stepping rate is near a resonant mechanical frequency of the system, resulting in failure to start.
Accordingly, it is a principal object of the present invention to provide method and apparatus to reliably start a reluctance motor from standstill and accelerate the motor to a desired speed, without the need for a shaft position sensor.
It is another object of the invention to provide a starting sequence adapted to be used with a switched reluctance motor using average total current as a feedback variable in closed-loop operation.
It is a further object of the invention to detect any failure to start a multiphase motor and to recommence the starting sequence in response to such failure.